On my Review Negative inventory screen, I have an item showing (-1), but no location in the location column. When I look on the Item record, none of the locations show (-1) at all. Any idea how this would have happened? / or how I can fix it?

Most likely was caused by Multi-location inventory being turned on after the account was already active. Look into a Distribute Inventory transaction. That might be able to fix it although maybe not with a negative value there.